Ciliated Protozoans

Although single-celled, there is nothing primitive or simple about these protists.

Not only are they large for single cells (some can be seen by the unaided eye), but they contain organelles that parallel in function the organs of multicellular creatures. In fact, some biologists consider the ciliates to be acellular (not cellular) rather than unicellular in order to emphasize that their "body" is far more elaborate in its organization than any cell out of which multicellular organisms are made.

The minichromosomes then replicate until as many as 20 million (2 x 107) may be present in the macronucleus.

Two parents come together and two parents separate. What kind of reproduction is that? you may well ask.

But the process they have been through is the very essence of sexual reproduction — genetic recombination. The "offspring" are not the same as the parents. They are new individuals and their macronucleus will soon reflect that fact.

Curiously, though, they have also become identical twins. Each parent formed two identical haploid nuclei — gave one away and kept the other. Thus when the cells separate, their new diploid micronuclei are identical. As the twins begin asexual reproduction (by fission), they are the founders of a clone.

Ciliated protozoans have been the source of several important discoveries in biology, for example: